Intermetallic compounds of rare-earth elements and transition metals have an ability to allow hydrogen to infiltrate into their crystal lattice, that is, the ability to store and release hydrogen in and out of the alloy. This property is employed in a variety of applications, such as hydrogen cells that make use of hydrogen storage alloys as typified by LaNi5. In rare-earth sintered magnet-related applications, the same property is used as a size reduction method for R2Fe14B-based alloys, and in the hydrogenation disproportionation desorption recombination (HDDR) process for producing R2Fe14B-based bonded magnets (JP-A 3-129702).
However, hydrogen storage and release in an alloy or magnet causes hydrogen embrittlement. Thus, when a motor or other device that uses a rare-earth sintered magnet is employed in a hydrogen atmosphere, the rare-earth sintered magnet undergoes hydrogen embrittlement, resulting in breaking, cracking or powdering of the magnet material.
Currently available rare-earth sintered magnets include R2Fe14B-based, SmCo5-based and Sm2Co17-based magnets. Generally, a 1:5 crystal structure has a lower plateau pressure for hydrogen than a 2:17 crystal structure, and a 2:7 crystal structure has a lower plateau pressure than a 1:5 crystal structure. Thus, a rare earth-rich (abbreviated below as “R-rich”) alloy tends to retain hydrogen more easily and to be more readily subject to hydrogen embrittlement.
R2Fe14B-based magnets have a R-rich phase in the magnet, as a result of which they readily undergo hydrogen embrittlement in hydrogen atmospheres at pressures of 0.1 MPa or less, leading to breaking, cracking or degradation of the magnet material. R2Fe14B-based magnets are usually given a surface treatment, such as plating or resin coating, to improve corrosion resistance, although such treatment is not a means for preventing hydrogen embrittlement. A solution is proposed in JP-A 2000-285415, which describes a method for including a hydrogen storage alloy in the surface treatment film on R2Fe14B-based magnets. R2Fe14B-based magnets produced by this method do not undergo hydrogen embrittlement in hydrogen atmospheres at pressures of 0.1 MPa or less, but they apparently undergo hydrogen embrittlement in hydrogen atmospheres at higher pressures than this, leading to breaking, cracking or degradation of the magnet material.
SmCo5-based magnets, like R2Fe14B-based magnets, have a R-rich phase. In addition, the SmCo5 phase, which is the main phase, has a plateau pressure of about 0.3 MPa. Hence, in a hydrogen atmosphere at a pressure higher than 0.3 MPa, hydrogen embrittlement occurs, resulting in breaking, cracking or degradation of the magnet material.
In Sm2Co17-based magnets, the main phase is a 2:17 phase. Unlike R2Fe14B-based magnets and SmCo5-based magnets, Sm2Co17-based magnets are not R-rich and do not contain an R-rich phase. Hence, they are not readily subject to hydrogen embrittlement. However, in hydrogen atmospheres at pressures higher than 1 MPa, Sm2Co17-based magnets too, like the other types of rare-earth sintered magnets mentioned above, undergo hydrogen embrittlement, resulting in breaking, cracking or degradation of the magnet material.